But no decision is expected soon, and Obama appears to have softened his 'red line' threat over chemical weapons use by the Assad regime.

A man runs with one of the injured after an explosion in Damascus, the Syrian… (Syrian Arab News Agency,…)

WASHINGTON — The White House is considering providing weapons to the Syrian rebels, officials said Tuesday, but no decision is imminent and President Obama seemed to soften his public threats to the Syrian government over its alleged use of chemical weapons.

A decision to supply weapons would mark a reversal for the Obama administration, which has resisted repeated proposals to deepen its involvement in Syria's 2-year-old conflict, which, according to the United Nations, has killed more than 70,000 people, mostly civilians.

The contradictory signals reflect the growing dilemma for Obama. His administration is struggling to show resolve and build pressure on Syrian President Bashar Assad and Syria's Russian and Iranian allies, even as the White House seeks to avoid getting drawn into another war in the Muslim world.

The administration also is considering supplying nonlethal military equipment such as armor, communications gear, night-vision goggles and vehicles to Syrian insurgent groups, officials said.

"We continue to consider all other possible options that would accomplish our objective of hastening a political transition," said Caitlin Hayden, spokeswoman for the National Security Council.

Another senior U.S. official, who declined to be identified when speaking on a sensitive issue, said arming the rebels was "on the table — and has been for some time — but there's a ways to go before any decision is made."

At a White House news conference, Obama appeared to raise the bar for U.S. military involvement. If evidence conclusively shows that Syrian forces used poison gas, he said, he "would rethink a range" of retaliatory options that might not include military action. He also suggested that he would require international collaboration for any military operation, saying the burden of a response is not America's alone.

"What's happening in Syria is a blemish on the international community generally," he said. Confirmed use of poison gas by Syrian forces would be "a game changer not simply for the United States, but for the international community."

Obama was asked directly if that meant his administration would take military action.

"By 'game changer' I mean that we would have to rethink the range of options that are available to us," he said. If the White House obtains conclusive proof of poison gas use, "that means that there's some options that we might not otherwise exercise that we would ... strongly consider."

The president's comments came after a huge bomb exploded Tuesday in a busy central square in Damascus, the Syrian capital. The official Syrian news agency reported that at least 13 people had been killed and more than 70 injured. There was no immediate claim of responsibility.

The explosion occurred a day after an apparent car bomb targeted the convoy of Syrian Prime Minister Wael Halqi in Mezzeh, a district of Damascus. Halqi was unharmed, the government said, but reports indicated that several people were killed.

The two bombings, after a relative lull in attacks in the capital, have rattled nerves on the eve of a six-day holiday that begins Wednesday. Several mortar strikes also were reported Tuesday in Damascus.

"I guess all these days were only the calm before the storm," said a retired lawyer in Damascus. "God help us with what's coming tomorrow."

Rebels seeking to overthrow the government frequently use car bombs and have fired mortar rounds into civilian areas of the capital. Government artillery and warplanes also have targeted civilian districts where rebel presence is suspected.

Obama first warned Assad in August that use of chemical weapons, or transfer of such stockpiles to terrorist groups, would cross a "red line." He left ambiguous what kind of response he might take, although it seemed to imply U.S. military action.

But since the White House last week announced that U.S. intelligence agencies had concluded with varying degrees of confidence that sarin, a nerve gas, had been released in Syria, Obama has made it gradually clearer that he is far from embracing a military option.

Obama emphasized Tuesday that investigators had yet to determine if Syrian government forces intentionally used the gas, if it was accidentally released or if insurgents or other groups fired the weapons. The president said conclusive proof was necessary to win international support for action.

"If we end up rushing to judgment without hard, effective evidence, then we may find ourselves in the position where we can't mobilize the international community to support what we do," he said. "There may be objections even among some people in the region who are sympathetic with the opposition if we take action."

Obama is dispatching Secretary of State John F. Kerry to Moscow in the next few days to try to convince the Kremlin to end its support of the Assad regime.